Conventionally, in a position sensor or the like, an electromagnetic characteristic changing portion consisting of concave and convex grooves is formed on a scale; a magnetic field is applied to the electromagnetic characteristic changing portion; and an electromagnetic sensing element, such as a ferromagnetic thin-film magneto-resistive element, is used to detect fluctuation of magnetic flux caused by the electromagnetic characteristic changing portion. To detect the fluctuation of magnetic flux with a high sensibility, a bridge circuit can be made up of four magneto-resistive elements.
More particularly, as shown in FIG. 4(A), four magneto-resistive elements 10a through 10d are arranged along a graduation 14 consisting of concave and convex grooves formed on a scale 12. Then, when the magneto-resistive element 10a and the next magneto-resistive element 10c but one are simultaneously located at positions corresponding to a convex region 16 of the scale 12, the remaining two magneto-resistive elements 10b and 10d are located at positions corresponding to a concave region 18.
Furthermore, the magneto-resistive elements 10a through 10d constitute, as shown in FIG. 4(B), a bridge circuit 20 that serves as a detection circuit. More particularly, the cross-hatched magneto-resistive elements 10a and 10c that arrive simultaneously to the positions corresponding to the convex regions 16 compose two opposing sides of the bridge circuit 20. The magneto-resistive elements 10b and 10d that arrive to the positions corresponding to the concave regions 18 compose the other two opposing sides of the bridge circuit 20. In this event, if Ra through Rd denote electrical resistance of the magneto-resistive elements 10a through 10d, respectively, then an output E of the bridge circuit 20 can be represented by: EQU E.varies.Rb.times.Rd-Ra.times.Rc (1).
In the bridge circuit 20 made up in the manner described above, the scale 12 is formed by the concave and convex grooves consisting of the convex regions 16 and the concave regions 18. Accordingly, as shown in FIG. 5, the magnetic field (magnetic flux .PHI.) applied by a magnet 22 is fluctuated and deviated at the edge portions of the convex regions 16 of the scale 12. As a result, when the output of the bridge circuit 20 has a repetitious waveform as in the case of a position sensor, it is possible to carry out successfully a comparator processing for repeatedly digitizing the output of the bridge circuit 20.
However, when the electromagnetic characteristic changing portion(s) is/are formed singly or at spaces relative to the member to be detected to allow generation of so-called one-pulse waveform (1 bit) as in the case of, for example, detecting an original point or the like of the scale, small waves are frequently generated as a noise beside the fundamental wave due to the fluctuation of the magnetic field caused around the electromagnetic characteristic changing portions, which makes the comparator processing difficult.
Now, each of the magneto-resistive elements 10a through 10d constituting the bridge circuit 20 is passed over the electromagnetic characteristic changing portion formed of the convex portion 16 of the scale 12 as shown in FIGS. 6(A) through 6(H). By the way, for the electrical resistance Ra through Rd, the resistance values thereof are not changed when the magnetic field is applied to the element in a perpendicular direction, while the resistance values thereof are reduced when the magnetic field is fluctuated at the electromagnetic characteristic changing portion of the convex region 16 to apply a transversal component to the element. Accordingly, the output E obtained in Equation 1 varies as follows:
For the condition shown in FIG. 6(A), EQU Rb.times.Rd-Ra.times.Rc=0;
For the condition shown in FIG. 6(B), EQU Rb.times.Rd-Ra.times.Rc&lt;0
For the condition shown in FIG. 6(C), EQU Rb.times.Rd-Ra.times.Rc&gt;0;
For the condition shown in FIG. 6(D), EQU Rb.times.Rd-Ra.times.Rc&lt;0;
For the condition shown in FIG. 6(E), EQU Rb.times.Rd-Ra.times.Rc&lt;0;
For the condition shown in FIG. 6(F), EQU Rb.times.Rd-Ra.times.Rc&lt;0;
For the condition shown in FIG. 6(G), EQU Rb.times.Rd-Ra.times.Rc&gt;0;
For the condition shown in FIG. 6(H), EQU Rb.times.Rd-Ra.times.Rc=0.
As apparent from the above, a plurality of output signals are produced on the positive side or on the negative side of the reference voltage (zero point). Consequently, in a conventional method for detecting the electromagnetic characteristic changing portion, detection of the one-pulse waveform results in detection of two output waveforms having approximately equal peaks on the positive side or on the negative side, which can be a cause of detection errors.
On the other hand, as disclosed in, for example, Japanese Patent Publication No. 56-1567, there are some cases where the electromagnetic characteristic changing portion is formed by means of polarization and magneto-sensitive directions of two pairs of ferromagnetic magneto-resistive elements connected in series and orthogonally crossed each other, whereby a difference between the outputs of the two is detected. In such a case, however, the output waveform becomes larger in width and in area, which causes degraded accuracy of detection.